docs: add Japanese README

[eltociear]

What type of PR is this?

I created Japanese translated README.

Check the PR title.

• This PR title match the format: <type>(optional scope): <description>
• The description of this PR title is user-oriented and clear enough for others to understand.
• Attach the PR updating the user documentation if the current PR requires user awareness at the usage level. User docs repo

(Optional) Translate the PR title into Chinese.

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en:
zh(optional):

(Optional) Which issue(s) this PR fixes:

(optional) The PR that updates user documentation:

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.35%. Comparing base (59be92f) to head (259548a).
⚠️ Report is 86 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #946      +/-   ##
==========================================
+ Coverage   51.86%   53.35%   +1.48%     
==========================================
  Files         127      171      +44     
  Lines       10893    14146    +3253     
==========================================
+ Hits         5650     7548    +1898     
- Misses       4920     6503    +1583     
+ Partials      323       95     -228     

Flag	Coverage Δ
arm	45.37% <ø> (?)
macos-15	46.98% <ø> (?)
ubuntu-24.04-arm	45.37% <ø> (?)
ubuntu-latest	52.08% <ø> (?)
x86	52.08% <ø> (?)

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